1. Field of the Invention
This invention relates to a system and method for reducing angular geometric distortion in an imaging device.
2. Background of the Related Art
A number of diagnostic tools are used in the field of Interventional Cardiology to diagnose and treat heart disease. Among other things, Intravascular Ultrasonic (IVUS) imaging systems are used to estimate an extent of coronary artery plaque, allowing an interventionalist to detect heart disease, estimate the severity of the disease and its effect on blood flow, and choose appropriate therapies. One such system is a fluoroscopy system, which injects a radio-opaque contrast dye into the bloodstream, and then uses a real time x-ray imaging technique to watch as the contrast dye passes through the vascular tree. However, fluoroscopy systems do not image the vessel tissue, but rather the blood pool inside the vessel lumen.
In an attempt to obtain an image of the vessel wall, and not just the blood pool, ultrasonic imaging transducers are mounted at the distal end of catheters and positioned in the coronary arteries. This approach allows the visualization of the tissues of the artery walls, and more particularly, visualization of the atherosclerotic plaque that forms in these vessels. However, these images are compromised in that the assumed angular orientation of the ultrasonic transducer at the distal tip of the catheter is not, in fact, the actual orientation of the transducer.
More particularly, in many IVUS imaging systems, the proximal end of a torque cable is spun at a uniform velocity by an electric motor. For example, a motor rotated at approximately 1800 RPM generates a complete circular cross section at a rate of approximately 30 frames, or images, each second. However, a catheter has a number of bends and twists in its length necessary to get from the entry point into the human body to a point at which an image is acquired. One of the most common distortions generated by these types of IVUS systems is Non-Uniform Rotational Distortion (NURD).
NURD can be caused by a number of different sources, including, for example, friction between the spinning torque cable and the stationary sheath that encloses the cable, or the torque cable and transducer assembly being not perfectly cylindrically symmetrical, causing the cable to resist bending more at some angles than at other angles. When rotated, these asymmetries will cause the cable to store more energy in some angular orientations and then to release that energy as the cable is rotated past that angle. In either case, the angular velocity of the transducer varies with each cycle even if the motor maintains a constant speed at the proximal end of the catheter. In many situations, an absolute angular orientation of a particular lesion or section of a lesion may be critical in performing an accurate, timely diagnosis and prescribing appropriate treatment. Thus, it is important to substantially eliminate, or at least significantly reduce, NURD so that these measures of angular extent are accurate and reliable.
To this end, new IVUS systems have been proposed in which the catheter is manually rotated in either direction so as to produce an image of a sector of an artery, rather than spinning the catheter in a single direction. However, this change in rotational direction leads to another form of NURD caused by the “windup” and resulting “backlash” of the transducer when the rotational direction is changed. Electronically steered imaging systems produce images without rotating the transducers on the distal end of the catheter, and thus do not produce any rotational distortion. However, this comes at the expense of increased transducer, catheter, and imaging system complexity and cost.